1. Field of the Invention
The present invention relates to VSB (Vestigial SideBand) reception systems, and more particularly, to a VSB reception system with a frequency-domain equalizer.
2. Background of the Related Art
In currently used most of the digital transmission systems and the ATSC (Advanced Television Systems Committee) 8VSB transmission system suggested as digital TV transmission system for the USA, when a broadcasting signal is transmitted through air or a line, a digital TV receiver receives the broadcasting signal together with signals reflected at various reflectors. Since reflected components distort an original signal, the original signal can not be obtained from the received signal only. The components which distort the original signal are ghost or fading.
The signal distorted thus causes a bit detection error in the digital TV receiver, such that restoration of a whole picture is impossible, or a totally different picture is displayed.
In order to overcome this problem, an equalizer is used in the digital TV receiver for correcting the components (i.e., the ghost or the fading) that distort a signal transmitted between a transmission terminal and a reception terminal.
In the equalizers, there are a time-domain equalizer and frequency-domain equalizer, all of which serve to remove the components that distort the original signal.
The frequency-domain equalizer assumes an impulse response of a channel from a time domain signal, and converts the impulse response into a frequency domain signal. Then, the frequency-domain equalizer controls parameters of the frequency-domain equalizer so that the frequency domain signal distorted on the channel becomes an ideal frequency domain signal, for correcting the distortion. That is, after a received time domain signal is converted into a frequency domain signal, the fading components are corrected by a frequency domain equalizer. Then, the corrected frequency domain signal is converted into a time domain signal, again.
FIG. 1 illustrates a block diagram of a related art VSB reception system with a frequency domain equalizer, wherein the demodulating part 1 tunes to a desired channel from RF (Radio Frequency) signals received through an antenna, converts a RF signal on the tuned channel into an IF (Intermediate Frequency) signal, demodulates the IF signal in reverse of a VSB modulating system, and provides to the frequency domain equalizing part 2. Then, the frequency domain equalizing part 2 assumes the impulse response of the channel from the time domain signal received thus, converts the impulse response into a frequency domain signal, and corrects fading components thereof by controlling parameters.
However, once a signal with the fading components passes through the frequency domain equalizing part 2, though the fading components are corrected, an amplification of noise is occurred, too.
FIGS. 2A and 2B illustrate diagrams each showing a fading signal in a VSB signal, wherein FIG. 2A illustrates a channel impulse response having a main signal and three fading signals, and FIG. 2B illustrates a frequency spectrum of an ideal signal from a transmitter, and a frequency spectrum having fading, of a signal from a transmitter, in which an AWGN (additive white gaussian noise) is contained.
FIG. 3A illustrates a diagram showing a signal, and a noise spectrum before equalizing, and FIG. 3B illustrates a diagram showing a signal, and a noise spectrum after equalizing.
Referring to FIG. 3A, the signal before equalizing contains the AWGN in all bandwidths, and the signal containing the fading has a very small spectrum size in a particular frequency bandwidth. As shown in FIG. 3B, though the signal after the equalizing, having the fading corrected, shows an ideal signal form, a power of the AWGN is increased after equalizing. Such an increase of noise causes poor performance of a whole system.